Boolean Difference - What am I doing wrong?
First off, you can set the unit system to imperial instead of metric under Scene Properties > Units > Unit System > Imperial, and set the Length to Inches.



When you're using real system, do not change the scale value. Instead, press N and go into Dimensions. Here you can set the values. You can see the difference between scale and dimensions in this picture. You can also type in other Imperial units, such as ft. This will automatically put it into feet. Math operations such as *(
multiplication) or / (division). It's really handy.



Now add the cube. Again, use the dimensions panel to get the correct size of the door. Go into side view by pressing 3 on the numpad, press G, hold CTRL and bring it up to the floor. For the door zoom in so you can see the smaller squares and do the same. Make sure you're using the increment under snapping settings.



I tried exactly what you've done and ran into the same issue. Yet the solution is pretty simple! With the door selected, press TAB to go into edit mode, select the bottom face and drag it down by pressing G, then Z and dragging it down a little bit. Then press TAB again to leave the edit mode and press S and X to scale it on the sides.



... but that's just in case you come to something like this again. Otherwise the simplest solution would be to make it thicker and move the bottom a little bit right when you're scaling the door! :P

Then repeat what you did - add the Boolean modifier, select the door as the cutter, operation to Difference hit apply and you're done! You can now also delete the door if you want to.



If you also want to fix the topology after boolean operation (which I would highly recommend), select the corner of the door hole and the corner of the wall and press J. Repeat this on each side so you have quads all around the mesh. We don't want any ngons there!



Hopefully a bit longer and more detailed reply was worth it. Happy Blending! :)